Method and apparatus for establishing adjacency for a restarting router during convergence

ABSTRACT

A method and apparatus are disclosed for establishing adjacencies on a network, the method comprising, at a first node of the network, sending hello packets on the network and receiving hello packets from other nodes on the network on the basis of the received hello packets. The node then sends a link-state packet without adjacency information and without an overload bit set. The node then interrogates a link-state adjacency table and, when only one adjacency is listed in the link-state table, sends a further link-state packet with the adjacency information and the overload bit set. On convergence of a forward cache, the node sends a further link-state packet with adjacency information and without the overload bit set.

FIELD OF THE INVENTION

The present invention generally relates to network routing. Theinvention relates more specifically to a method and apparatus forestablishing adjacency for a restarting router during convergence.

BACKGROUND OF THE INVENTION

The approaches described in this section could be pursued, but are notnecessarily approaches that have been previously conceived or pursued.Therefore, unless otherwise indicated herein, the approaches describedin this section are not prior art to the claims in this application andare not admitted to be prior art by inclusion in this section.

Internet protocol relates to a network layer protocol in the TCP/IPstack offering a connectionless inter-network service. Various routingprotocols have been used to implement such networks. In particular, LinkState Protocols have proved popular in which data is transmitted usingLink State Packets. An example of such a protocol is the IntermediateSystem-to-Intermediate System (IS-IS) protocol.

A network comprises a plurality of nodes connected together. Some nodesrepresent end systems (such as printers, fax machines, telephones, PC'setc) whereas other nodes represent network devices (e.g. switches,routers etc). Data packets are sent around the network from a source toa destination in accordance with routing information shared among thenodes of the network. As the network comprises a plurality ofinterconnected nodes, the network is fairly robust. Should a node failfor any reason, the network dynamically configures to re-route data soas to avoid the failed node.

Discovery messages, such as Hello messages, provide a mechanism by whichnodes on a network may continually indicate their presence. The “Hello”packets are used to establish routing adjacencies between directlyconnected routers and for the purpose of exchanging routing informationpackets.

A link can be thought of as an interface on a router. The state of thelink is a description of that interface and of its relationship to itsneighboring routers. A description of the interface would include, forexample, the IP address of the interface, the mask, the type of networkit is connected to, the router connected to that network and so on. Thecollection of all these link-states form a link-state database. Linkstate protocols use a link state algorithm to build and calculate theshortest path to all known destinations. The algorithms themselves arequite complicated but the following provides a high level simplified wayof looking at the various steps of a link state algorithm. Uponinitialization or due to any changing routing information, a router willgenerate a link state advertisement. This advertisement represents thecollection of all link states on that router. All routers exchange linkstates by means of flooding. Each router that receives a link stateupdate, stores a copy in its link state database and then propagates theupdate to other routers. After the database of each router is completed,the router will calculate the shortest path tree to all designations anduse this information to form an IP routing table.

In IS-IS, when a link or a node fails and is subsequently repaired, therouters involved with the repaired part of the network then have tore-establish an IS-IS adjacency over that link. This is achieved by therouter(s) transmitting Hello packets and receiving in response Hellopackets from their neighboring nodes. The router then generates anadjacency table with information received from the neighboring nodes andupdates its Link-State Protocol data unit (LSP) and floods it throughoutthe network area. Link-state routing protocols mean that each router inthe area, after having received a LSP, will compute a Shortest PathFirst and update its IP routing table.

The LSP as sent by the router advertises adjacency informationregardless of whether the routers are effectively able to forwardtraffic. When a router or a link has been down, there are additionaldelays whilst the routers obtain full knowledge of the forwarding cacheand until the Border Gateway Protocol (BGP) has also converged (ifneeded). Until these occur, the router is not in a state to forward datasent to it for onward routing.

The IS-IS protocol already includes some mechanisms to prevent therouter being used as a transit node in another router's table until itsconvergence is finished. Typically the overload bit LSPDBOL is setwhilst the system is converging. At the same time the adjacency of theLSP is advertised. However, this allows a receiving router to computeroutes towards a re-starting node before such a node is ready for data.

When a router restarts, it originates its LSP with a sequence numberof 1. As there will be old versions of the old LSP for the restartingrouter already existing in the network, its neighbors will flood backthe old copy of the restarting router LSP (if any) with the lastsequence number used before the restart. LSPs are kept in link statedatabases for a significant amount of time. Therefore the LSP of arestarting router will survive in the database of other routers in thenetwork. On receipt of the old copy of the restarting router LSP, therestarting router can determine the next sequence number it has to useand regenerates its LSP with a sequence number higher than the onereceived on the old copy of its LSP. Such LSP exchange takes time anduntil it is finished all routers in the area may compute their SPF usingan old copy of the restarting router LSP. Therefore routers in thenetwork may route data to the restarting router before it is fullyconverged.

Based on the foregoing, there is a clear need for a method of preventingrouters in the network from routing data to a restarting router beforethe router has fully converged.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example, and not by wayof limitation, in the figures of the accompanying drawings and in whichlike reference numerals refer to similar elements and in which:

FIG. 1 is a block diagram of an embodiment of a network;

FIG. 2 shows an example of a format of a Link State Protocol packet;

FIG. 3 is a flow diagram which illustrates a high level overview of oneembodiment of a method for re-establishing adjacency;

FIG. 4 shows an example of an adjacency table showing the IS-IS Level 1and Level 2 LSP; and

FIG. 5 is a block diagram that illustrates a computer system upon whichan embodiment may be implemented.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A method and apparatus for establishing adjacency for a restartingrouter during convergence is described. In the following description,for the purposes of explanation, numerous specific details are set forthin order to provide a thorough understanding of the present invention.It will be apparent, however, to one skilled in the art that the presentinvention may be practiced without these specific details. In otherinstances, well-known structures and devices are shown in block diagramform in order to avoid unnecessarily obscuring the present invention.

Embodiments are described herein according to the following outline:

-   -   General Overview    -   Structural and Functional Overview    -   Method of routing data to a restarting router during convergence    -   Implementation Mechanisms—Hardware Overview    -   Extensions and Alternatives

1.0 GENERAL OVERVIEW

The needs identified in the foregoing Background, and other needs andobjects that will become apparent for the following description, areachieved in the present invention, which comprises, in one aspect, amethod for establishing adjacencies on a network. A first node of thenetwork, for which adjacencies need to be established (for instancebecause the node is re-starting) sends hello packets out onto thenetwork. In response, the node receives hello packets from other nodeson the network on the basis of the hello packets sent by the first node.The first node then sends a link-state packet without adjacencyinformation and without an overload bit set. The first node theninterrogates a link-state adjacency table and, when only one adjacencyis listed in the link-state table, sends a further link-state packetwith the adjacency information and the overload bit set. When a forwardcache has converged, the node sends a further link-state packet withadjacency information and without the overload bit set.

In other aspects, the invention encompasses a computer apparatus and acomputer-readable medium configured to carry out the foregoing steps.

2.0 STRUCTURAL AND FUNCTIONAL OVERVIEW

One example of a networking protocol is the IntermediateSystem-to-Intermediate System (IS-IS) protocol. This protocol uses LinkState PDUs (LSPs) which enable fast convergence with large scalability.IS-IS will be used to illustrate the method and apparatus although it isnot essential that this protocol be used.

An internetwork comprises a plurality of interconnected sites. Trafficbetween sites is routed from the source to a destination via nodes ofthe network. Due to various factors (for instance excessive networktraffic, hardware failure or software failure), nodes may enter afailure mode, during which time data routed to that node is not routedonwards by that node.

FIG. 1 shows a simple block diagram illustrating an internetwork. In thenetwork shown, the network is divided administratively into two areas 2,4. Each area includes at least one intermediate system (IS). An exampleof an IS is a router. Attached to any of the routers are hosts known asEnd Systems (ESs). Generally a two level hierarchy is adopted. Routingwithin an area is referred to as level 1 routing. Routing between areasis referred to as level 2 routing. A level 2 IS (such as IS4) keepstrack of the paths to destination areas. A level 1 IS (e.g. IS1) keepstrack of the routing within its own area. For a packet destined foranother area, a level 1 IS sends the packet to the nearest level 2 IS inits own area (e.g. IS4), regardless of the destination area. The packettravels via level 2 routing to the destination area where it then maytravel via level 1 routing to the destination.

To maintain the integrity of the network, each node of the network needsto be continually informed of another node of the network to enablerouting. This is generally achieved by nodes emitting discovery messages(known as Hellos) and monitoring for responses to these Hello messages.

Each node includes a link state database which includes link stateinformation for the network. Routers send Hello packets out to allinterfaces to discover neighbors and establish adjacencies. Routerssharing a common data link will become neighbors if their Hello packetscontain information that meets the criteria for forming an adjacency.The main criteria are matching authentication, IS type and MTU size.Routers then build a link state packet (LSP) based upon their localinterfaces and prefixes learnt from other adjacent routers. Generallythe routers then flood their LSPs to all adjacent neighbors except theneighbor from who they received the same LSP. The routers then constructa link state database from these LSPs. A shortest path tree (SPT) isthen calculated by each IS and from this SPT a routing table is built.

An example of a LSP packet format is shown in FIG. 2. As can be seenwith reference to FIG. 2, the LSP includes the following fields:

-   -   Intradomain routing protocol discriminator—This is the network        layer identifier. A binary value of 10000011 indicates that the        network is an IS-IS network.    -   Length indicator—This is the length of the fixed header.    -   Version/Protocol ID ext—The version number of the protocol and        the extension number of the protocol, if applicable.    -   ID length—Length of the system ID field; this is the same for        all nodes in the domain.    -   PDU Type—Protocol Data Unit (PDU) types (e.g. Level 1 IS and        level 2 IS)    -   Version—The version number of the protocol.    -   Reserved—unused at the time of writing.    -   Maximum area address—Number of address areas permitted for the        IS area.    -   PDU Length—Length of the entire PDU, fixed header and any        Type-Length-Value (TLVs) fields.    -   Remaining Lifetime—Time in seconds before the LSP expires.    -   LSP ID—System ID, pseudocode and LSP fragmentation number.    -   Sequence number—updated sequentially by the originator each time        an LSP is sent.    -   Checksum—Computed from Source ID to end of PDU.    -   P—Partition—indicates whether originator of LSP supports        partition repair.    -   ATT—Attached—when any bit set, indicates the originator is        attached to another area using a referred metric.    -   LSPDBOL—LSP Database Overload. When set, this indicates the        originator's LSP database is overloaded and should be        circumvented in path calculations to other destinations.    -   IS Type—indicates Level 1 or Level 2 IS.    -   Type length fields—variable fields which store any TLV        information.

Where a node fails and is then subsequently repaired, it will benecessary for that node to acquire the LSPs from its neighbors so thatit can rebuild its link state database. While the node acquires the LSPsfrom neighboring nodes, there will be a period of time during which thenode's link state database is not complete. Clearly this may causeproblems if a packet is routed to that node since the node may not yethave received information relating to how a packet should be routed. Itis therefore important to avoid sending data to a node whilst it is inthis transition state, but it is desirable to be able to transit datavia this node but not via all its interfaces.

3.0 METHOD FOR ESTABLISHING ADJACENCY FOR A RESTARTING ROUTER DURINGCONVERGENCE

FIG. 3 shows a flow diagram which illustrates a high level overview ofone embodiment of a method for re-establishing adjacency in a network.First (301) it establishes that adjacency establishment is needed. Thiscan be based on a number of determining factors, for instance all theentries in the link state database having no remaining lifetime, theknowledge that a Line Card (LC) Forwarding Information Base (FIB) isbeing downloaded, a router restart or a LC restart. Once the node hasestablished that adjacency establishment is needed, the node transmitsHello packets through the network (302). These Hello messages are usedby routers to detect neighbors and form adjacencies. The router thenmonitors for any received Hello packets (303). These Hello packets arereceived from network nodes that have received the Hello packets sent bythe router.

On receipt of the Hello packets, the router generates a link statepacket (304). The LSP is generated (304) without setting the overloadbit LSPDBOL and without including the adjacency information. Typicallyin IS-IS the adjacency information is advertised in the field TLV-2 orTLV-22. The field is omitted when no adjacency information isadvertised. This LSP is then flooded throughout the network (305).

By advertising a new LSP without the adjacency information, the wholenetwork is informed that the node is not ready to receive traffic fromthe particular adjacency. If the node under repair has undergone anevent which means that the node does not know the next sequence numberto use, this also allows the node to capture the correct sequence numberthat it should be using. The node under repair generates a LSP with asequence number of 1. When a receiving neighbor node receives the LSPwith a sequence number of 1, the receiving neighbor node will transmitback to the node under repair an LSP with a higher sequence number andthe node under repair will then know which is the next sequence numberto be used. This next sequence number is then used in subsequent LSPgeneration.

On receipt of subsequent hello packets, the node under repair generatesan adjacency table (310), an example of which is shown in FIG. 4. Therouter then determines (306) if the newly established adjacency is theonly one in the IS-IS adjacency table or if there are other adjacenciesbut for IS-IS levels other than the newly established one. If the onlyadjacency in the adjacency table is the adjacency of the restarting nodethis indicates that the router has been isolated from the rest of thenetwork and therefore BGP sessions have got lost. If the newlyestablished adjacency is the only one in the adjacency table, the LSP issent with the overload bit LSPDBOL set and with the new adjacencyadvertised (307). This allows routers in the area to route around therouter under repair (owing to the overload bit being set) but allows BGPsessions to be established with the router under repair (owing to theadjacency information being advertised). This allows for BGPconvergence.

If there is more than one adjacency in the adjacency table thisindicates that the router is connected to the rest of the network andthat BGP sessions have stayed alive. Therefore the restarting node doesnot need to send a new LSP with the adjacency information advertised butthe overload bit set.

In either case, when the forwarding cache has converged (308), therouter then floods a new version of its LSP throughout the network (309)with the adjacency information and without the overload bit LSPDBOL set.Cache convergence may be indicated by a signal from the process incharge of the population of the cache.

Steps 306 and 307 are used mainly when a router is restarting or whenthe router has been completely disconnected from the rest of thenetwork. Steps 306 and 308 are mainly used when the node under repairrequires convergence of the IGP, BGP and forwarding cache prior toaccepting and sending traffic from or to a newly restarted interface ofthe node under repair.

The method described has the advantage that the IS-IS protocol behaviordoes not require any modification. The modifications relate to how arouter re-originates its Link-State PDU (LSP) with the new adjacencyinformation.

4.0 IMPLEMENTATION MECHANISMS—HARDWARE OVERVIEW

FIG. 5 is a block diagram that illustrates a computer system 500 uponwhich an embodiment of the invention may be implemented. The preferredembodiment is implemented using one or more computer programs running ona network element such as a router device. Thus, in this embodiment, thecomputer system 500 is a router.

Computer system 500 includes a bus 502 or other communication mechanismfor communicating information, and a processor 504 coupled with bus 502for processing information. Computer system 500 also includes a mainmemory 506, such as a random access memory (RAM), flash memory, or otherdynamic storage device, coupled to bus 502 for storing information andinstructions to be executed by processor 504. Main memory 506 also maybe used for storing temporary variables or other intermediateinformation during execution of instructions to be executed by processor504. Computer system 500 further includes a read only memory (ROM) 508or other static storage device coupled to bus 502 for storing staticinformation and instructions for processor 504. A storage device 510,such as a magnetic disk, flash memory or optical disk, is provided andcoupled to bus 502 for storing information and instructions.

A communication interface 518 may be coupled to bus 502 forcommunicating information and command selections to processor 504.Interface 518 is a conventional serial interface such as an RS-232 orRS-422 interface. An external terminal 512 or other computer systemconnects to the computer system 500 and provides commands to it usingthe interface 518. Firmware or software running in the computer system500 provides a terminal interface or character-based command interfaceso that external commands can be given to the computer system.

A switching system 516 is coupled to bus 502 and has an input interface514 and an output interface 519 to one or more external networkelements. The external network elements may include a local network 522coupled to one or more hosts 524, or a global network such as Internet528 having one or more servers 530. The switching system 516 switchesinformation traffic arriving on input interface 514 to output interface519 according to pre-determined protocols and conventions that are wellknown. For example, switching system 516, in cooperation with processor504, can determine a destination of a packet of data arriving on inputinterface 514 and send it to the correct destination using outputinterface 519. The destinations may include host 524, server 530, otherend stations, or other routing and switching devices in local network522 or Internet 528.

The invention is related to the use of computer system 500 forre-establishing adjacencies on a network. According to one embodiment ofthe invention, this is provided by computer system 500 in response toprocessor 504 executing one or more sequences of one or moreinstructions contained in main memory 506. Such instructions may be readinto main memory 506 from another computer-readable medium, such asstorage device 510. Execution of the sequences of instructions containedin main memory 506 causes processor 504 to perform the process stepsdescribed herein. One or more processors in a multi-processingarrangement may also be employed to execute the sequences ofinstructions contained in main memory 506. In alternative embodiments,hard-wired circuitry may be used in place of or in combination withsoftware instructions to implement the invention. Thus, embodiments ofthe invention are not limited to any specific combination of hardwarecircuitry and software.

The term “computer-readable medium” as used herein refers to any mediumthat participates in providing instructions to processor 504 forexecution. Such a medium may take many forms, including but not limitedto, non-volatile media, volatile media, and transmission media.Non-volatile media includes, for example, optical or magnetic disks,such as storage device 510. Volatile media includes dynamic memory, suchas main memory 506. Transmission media includes coaxial cables, copperwire and fiber optics, including the wires that comprise bus 502.Transmission media can also take the form of acoustic or light waves,such as those generated during radio wave and infrared datacommunications.

Common forms of computer-readable media include, for example, a floppydisk, a flexible disk, hard disk, magnetic tape, or any other magneticmedium, a CD-ROM, any other optical medium, punch cards, paper tape, anyother physical medium with patterns of holes, a RAM, a PROM, and EPROM,a FLASH-EPROM, any other memory chip or cartridge, a carrier wave asdescribed hereinafter, or any other medium from which a computer canread.

Various forms of computer readable media may be involved in carrying oneor more sequences of one or more instructions to processor 504 forexecution. For example, the instructions may initially be carried on amagnetic disk of a remote computer. The remote computer can load theinstructions into its dynamic memory and send the instructions over atelephone line using a modem. A modem local to computer system 500 canreceive the data on the telephone line and use an infrared transmitterto convert the data to an infrared signal. An infrared detector coupledto bus 502 can receive the data carried in the infrared signal and placethe data on bus 502. Bus 502 carries the data to main memory 506, fromwhich processor 504 retrieves and executes the instructions. Theinstructions received by main memory 506 may optionally be stored onstorage device 510 either before or after execution by processor 504.

Communication interface 518 also provides a two-way data communicationcoupling to a network link 520 that is connected to a local network 522.For example, communication interface 518 may be an integrated servicesdigital network (ISDN) card or a modem to provide a data communicationconnection to a corresponding type of telephone line. As anotherexample, communication interface 518 may be a local area network (LAN)card to provide a data communication connection to a compatible LAN.Wireless links may also be implemented. In any such implementation,communication interface 518 sends and receives electrical,electromagnetic or optical signals that carry digital data streamsrepresenting various types of information.

Network link 520 typically provides data communication through one ormore networks to other data devices. For example, network link 520 mayprovide a connection through local network 522 to a host computer 524 orto data equipment operated by an Internet Service Provider (ISP) 526.ISP 526 in turn provides data communication services through theworldwide packet data communication network now commonly referred to asthe “Internet” 528. Local network 522 and Internet 528 both useelectrical, electromagnetic or optical signals that carry digital datastreams. The signals through the various networks and the signals onnetwork link 520 and through communication interface 518, which carrythe digital data to and from computer system 500, are exemplary forms ofcarrier waves transporting the information.

Computer system 500 can send messages and receive data, includingprogram code, through the network(s), network link 520 and communicationinterface 518. In the Internet example, a server 530 might transmit arequested code for an application program through Internet 528, ISP 526,local network 522 and communication interface 518. In accordance withthe invention, one such downloaded application provides for routing datato a restarting router during convergence as described herein.

The received code may be executed by processor 504 as it is received,and/or stored in storage device 510, or other non-volatile storage forlater execution. In this manner, computer system 500 may obtainapplication code in the form of a carrier wave.

5.0 EXTENSIONS AND ALTERNATIVES

In the foregoing specification, the invention has been described withreference to specific embodiments thereof. It will, however, be evidentthat various modifications and changes may be made thereto withoutdeparting from the broader spirit and scope of the invention. Thespecification and drawings are, accordingly, to be regarded in anillustrative rather than a restrictive sense.

1. A method of establishing adjacencies on a network, the methodcomprising, at a first node of the network, sending one or more hellopackets on the network; receiving one or more hello packets from othernodes on the network on the basis of the sent hello packets; in responseto receiving the one or more hello packets, sending a first link-statepacket in Intermediate System-to-Intermediate System protocol; whereinthe first link-state packet includes a field for an overload bit;wherein the overload bit in said field is not set; wherein the firstlink-state packet comprises no adjacency information; interrogating alink-state adjacency table and, when only one adjacency is listed in thelink-state table, sending a second link-state packet in IntermediateSystem-to-Intermediate System protocol; wherein the second link-statepacket comprises adjacency information and an overload bit that is set;and on convergence of a forward cache, sending a further link-statepacket in Intermediate System-to-Intermediate System protocol; whereinthe further link-state packet comprises adjacency information and anoverload bit that is not set.
 2. A method according to claim 1 whereinthe method is initiated when the first node is in a restart node.
 3. Amethod according to claim 2 wherein the restart node is a line cardrestart, a router restart or a download of a forwarding informationbase.
 4. A method according to claim 1 wherein the network usesIntermediate System-to-Intermediate System protocol and wherein theadjacency information is advertised in a Type Length Variable field ofthe link-state packet.
 5. A method of re-establishing adjacency in aninter-networked system, the method comprising: i) determining thatadjacency establishment is required; ii) transmitting a message todiscover neighboring network elements; iii) receiving one or moremessages from neighboring network elements; and iv) in response to theone or more received messages, generating a first link-state packet inIntermediate System-to-Intermediate System protocol; wherein the firstlink-state packet includes a field for an overload bit; wherein theoverload bit in said field is not set; wherein the first link-statepacket comprises no adjacency information; v) sending the firstlink-state packet; vi) interrogating a link-state adjacency table and,when only one adjacency is listed in the link-state table, sending asecond link-state packet in Intermediate System-to-Intermediate Systemprotocol; wherein the second link-state packet comprises adjacencyinformation and an overload bit that is set; and vii) on convergence ofa forward cache, sending a further link-state packet in IntermediateSystem-to-Intermediate System protocol; wherein the further link-statepacket comprises adjacency information and an overload bit set that isnot set.
 6. A computer-readable storage medium storing one or moresequences of instructions for establishing adjacencies on a network,which instructions, when executed by one or more processors, cause theone or more processors to carry out the steps of: sending one or morehello packets on the network; receiving one or more hello packets fromother nodes on the network on the basis of the sent hello packets; inresponse to receiving the one or more hello packets, sending a firstlink-state packet in Intermediate System-to-Intermediate Systemprotocol; wherein the first link-state packet includes a field for anoverload bit; wherein the overload bit in said field is not set; whereinthe first link-state packet comprises no adjacency information;interrogating a link-state adjacency table and, when only one adjacencyis listed in the link-state table, sending a second link-state packet inIntermediate System-to-Intermediate System protocol; wherein the secondlink-state packet comprises adjacency information and an overload bitthat is set; and on convergence of a forward cache, sending a furtherlink-state packet in Intermediate System-to-Intermediate Systemprotocol; wherein the further link-state packet comprises adjacencyinformation and an overload bit that is not set.
 7. A computer-readablestorage medium as claimed in claim 6 further comprising instructionswhich, when executed by the one or more processors, cause the one ormore processors to carry out the steps of: initiating the method when ina restart node.
 8. A computer-readable storage medium as claimed inclaim 6 further comprising instructions which, when executed by the oneor more processors, cause the one or more processors to carry out thesteps of: initiating the method when in a restart mode comprising one ormore of the following: a line card restart, a router restart or adownload of a forwarding information base.
 9. A computer-readablestorage medium as claimed in claim 6 wherein the network usesIntermediate System-to-Intermediate System protocol and wherein theadjacency information is advertised in a Type Length Variable field ofthe link-state packet.
 10. A computer-readable storage medium storingone or more sequences of instructions for re-establishing adjacency inan inter-networked system, which instructions, when executed by one ormore processors, cause the one or more processors to carry out the stepsof: i) determining that adjacency establishment is required; ii)transmitting a message to discover neighboring network elements; iii)receiving one or more messages from neighboring network elements; andiv) in response to the one or more received messages, generating a firstlink-state packet in Intermediate System-to-Intermediate Systemprotocol; wherein the first link-state packet includes a field for anoverload bit; wherein the overload bit in said field is not set; whereinthe first link-state packet comprises no adjacency information; v)sending the first link-state packet; vi) interrogating a link-stateadjacency table and, when only one adjacency is listed in the link-statetable, sending a second link-state packet in IntermediateSystem-to-Intermediate System protocol; wherein the second link-statepacket comprises adjacency information and an overload bit that is set;and vii) on convergence of a forward cache, sending a further link-statepacket in Intermediate System-to-Intermediate System protocol; whereinthe further link-state packet comprises adjacency information and anoverload bit set that is not set.
 11. Apparatus for establishingadjacencies on a network, the apparatus comprising: means for sendingone or more hello packets on the network; means for receiving one ormore hello packets from other nodes on the network on the basis of thesent hello packets; means for, in response to receiving the one or morehello packets, sending a first link-state packet in IntermediateSystem-to-Intermediate System protocol; wherein the first link-statepacket includes a field for an overload bit; wherein the overload bit insaid field is not set; wherein the first link-state packet comprises noadjacency information; means for interrogating a link-state adjacencytable and, when only one adjacency is listed in the link-state table,sending a second link-state packet in IntermediateSystem-to-Intermediate System protocol; wherein the second link-statepacket comprises adjacency information and an overload bit that is set;and on convergence of a forward cache, means for sending a furtherlink-state packet in Intermediate System-to-Intermediate Systemprotocol; wherein the further link-state packet comprises adjacencyinformation and an overload bit that is not set.
 12. Apparatus forre-establishing adjacency in an inter-networked system, the apparatuscomprising: i) means for determining that adjacency establishment isrequired; ii) means for transmitting a message to discover neighboringnetwork elements; iii) means for receiving one or more messages fromneighboring network elements; and iv) means for in response to the oneor more received messages, generating a first link-state packet inIntermediate System-to-Intermediate System protocol; wherein the firstlink-state packet includes a field for an overload bit; wherein theoverload bit in said field is not set; wherein the first link-statepacket comprises no adjacency information; v) means for sending thefirst link-state packet; vi) means for interrogating a link-stateadjacency table and, when only one adjacency is listed in the link-statetable, sending a second link-state packet in IntermediateSystem-to-Intermediate System protocol; wherein the second link-statepacket comprises adjacency information and an overload bit that is set;and vii) on convergence of a forward cache, means for sending a furtherlink-state packet in Intermediate System-to-Intermediate Systemprotocol; wherein the further link-state packet comprises adjacencyinformation and an overload bit set that is not set.
 13. An apparatusfor establishing adjacencies on a network, the apparatus comprising: anetwork interface that is coupled to the network for receiving one ormore packet flows therefrom; a processor; one or more stored sequencesof instructions which, when executed by the processor, cause theprocessor to carry out the steps of: sending one or more hello packetson the network; receiving one or more hello packets from other nodes onthe network on the basis of the sent hello packets; in response toreceiving the one or more hello packets, sending a first link-statepacket in Intermediate System-to-Intermediate System protocol; whereinthe first link-state packet includes a field for an overload bit;wherein the overload bit in said field is not set; wherein the firstlink-state packet comprises no adjacency information; interrogating alink-state adjacency table and, when only one adjacency is listed in thelink-state table, sending a second link-state packet in IntermediateSystem-to-Intermediate System protocol; wherein the second link-statepacket comprises adjacency information and an overload bit that is set;and on convergence of a forward cache, sending a further link-statepacket in Intermediate System-to-Intermediate System protocol; whereinthe further link-state packet comprises adjacency information and anoverload bit that is not set.
 14. An apparatus for re-establishingadjacency in an inter-networked system, the apparatus comprising: anetwork interface that is coupled to the network for receiving one ormore packet flows therefrom; a processor; one or more stored sequencesof instructions which, when executed by the processor, cause theprocessor to carry out the steps of: i) determining that adjacencyestablishment is required; ii) transmitting a message to discoverneighboring network elements; iii) receiving one or more messages fromneighboring network elements; and iv) in response to the one or morereceived messages, generating a first link-state packet in IntermediateSystem-to-Intermediate System protocol; wherein the first link-statepacket includes a field for an overload bit; wherein the overload bit insaid field is not set; wherein the first link-state packet comprises noadjacency information; v) sending the first link-state packet; vi)interrogating a link-state adjacency table and, when only one adjacencyis listed in the link-state table, sending a second link-state packet inIntermediate System-to-Intermediate System protocol; wherein the secondlink-state packet comprises adjacency information and an overload bitthat is set; and vii) on convergence of a forward cache, sending afurther link-state packet in Intermediate System-to-Intermediate Systemprotocol; wherein the further link-state packet comprises adjacencyinformation and an overload bit set that is not set.